A radar sensor is a sensing device for measuring information on a distance, velocity, and an angle by transmitting a radio wave such as a microwave, and receiving a reflection signal reflected from a target.
Such a radar sensor measures the target information by using various radar waveforms of a pulsed Doppler radar, a frequency modulated continuous wave (hereinafter referred to as “FMCW”) radar, a stepped-frequency continuous wave (hereinafter referred to as “SFCW”) radar, a frequency shift keying (hereinafter referred to as “FSK”) radar, and the like.
Generally, the pulse Doppler radar is used as a long-range detection radar, and the FMCW/SFCW/FSK radars are used for short-range detection.
Recently, the radar sensor has been applied to vehicle radar devices to prevent collision during driving and to support safe driving.
For example, FIG. 1 is a block diagram of a radar device using an FMCW radar waveform.
As shown in FIG. 1, a radar device 10 according to the related art includes: an antenna unit 11 for transmitting a radar signal to a periphery of a vehicle and receiving a signal reflected from another vehicle; a radio frequency (RF) unit 12 for generating the transmission signal, converting frequencies of the transmission signal and the reception signal, and amplifying the reception signal; and a digital unit 13 for generating a control signal to generate the transmission signal and determining whether a collision with another vehicle occurs or not based on radar detection information, which is acquired by signal-processing the reception and includes a distance to a target, a velocity of the target, and an angle of the target.
The RF unit 12 includes: a voltage controlled oscillator 21 for outputting the transmission signal, which has a desired oscillation frequency, according to the control signal of the digital unit 13; a low noise amplifier 22 for amplifying the signal received by the antenna unit 11 and attenuating noise; a power divider 23 for distributing the transmission signal generated by the voltage controlled oscillator 21; a pair of mixers 24 for mixing the reception signal outputted from the low noise amplifier 22 with in-phase components and quadrature components of the signal distributed by the power divider 22 to identify the in-phase components and the quadrature components of the reception signal; a pair of gain amplifiers 25 for amplifying gains of the respective components outputted from the pair of mixers 24; and a pair of bandpass filters for filtering the amplified respective components over a predetermined frequency band to remove noise.
The digital unit 13 may include: a signal processing unit 31 for outputting the control signal to generate the transmission signal and signal-processing the reception signal; a DAC 32 for converting the control signal in the form of a digital signal into an analog signal; and an ADC 33 for converting the reception signal in the form of an analog signal into a digital signal.
Since the radar device 10 having the above configuration uses a frequency band which is set to a 77 GHz band or a 24 GHz band, if the radar devices 10 are located in the same area, frequency interference occurs because the same frequency is used.
Accordingly, techniques are developed for avoiding the frequency interference by previously classifying and assigning a pseudo noise code (hereinafter referred to as “PN code”), a Barker code and the like over each user and applying frequency hopping patterns and time hopping patterns to the codes.
In Korean Patent Registration No. 10-1135982 (published on Apr. 17, 2012, hereinafter referred to as “patent document 1”), Korean Patent Registration No. 10-1348548 (published on Jan. 16, 2014, hereinafter referred to as “patent document 2”) and the like, there is disclosed a frequency interference cancellation technique of a radar sensor according to the related art.
For example, FIGS. 2 to 4 are views illustrating a method of cancelling frequency interference of a radar according to the related art.
FIG. 2 shows a state in which time synchronization is perfectly matched when the frequency interference is canceled by using a frequency hopping scheme in the FMCW radar according to the related art. In addition, FIG. 3 shows a state in which a noise level is increased, and FIG. 4 shows a state in which a ghost target is generated in the case that the time synchronization is not matched.
The time/frequency transmission signal used by the FMCW radar sensor can be used by previously assigning the transmission signal to each user based on a specific code.
Therefore, as shown in FIG. 2, when different frequency hopping patterns are applied to radars, there is a premise that the time synchronization between radar sensors has to be perfectly matched.
As a result, when the time synchronization is not matched, there is represented an interference characteristic in which the noise level increases as the interference signal appears to flow along the time axis, as shown in FIG. 2.
In addition, even if the radar sensor is separated by using the PN code and the Barker code as the time delay of the interference signal is continuously generated, the frequency interference as shown in FIG. 3 occurs again at a certain time point.
If the time delay becomes longer, as shown in FIG. 4, the frequency interference occurs with an interference characteristic of generating a ghost target appearing as if there is an actual target.
FIGS. 5 and 6 are views for explaining the cases shown in FIGS. 3 and 4 where the noise level is increased and the case where the ghost target is generated, respectively.
As shown in FIGS. 5 and 6, when the interference signal flows along the time axis as the time synchronization is not matched, two types of frequency interference characteristics, in which the noise level is increased or the ghost target is generated, occur repeatedly.
Therefore, depending on the time point at which the radar signal is received, the noise level may be increased, and the ghost target may be generated.
FIGS. 7 and 8 are views illustrating frequency interference characteristics in the case where the continuous wave frequency interference occurs, and in the case where different FMCW waveforms exist, respectively. FIGS. 9 and 10 are views illustrating the case where the noise level is increased, and the case where the ghost target is generated, respectively.
As shown in FIG. 7, when the continuous wave frequency interference occurs, the noise level is increased as the transmission signal and the interference signal are in a cross form.
However, in the case of continuous wave interference, even if the interference signal flows along the time axis as the time synchronization is not matched, the frequency of the interference signal is generated at a fixed position.
As shown in FIG. 8, radar sensors using different FMCW waveforms have an interference characteristic, in which the ghost target is not generated and only the noise level is increased.
Therefore, in frequency interference characteristics of the same type of radar systems, when the time synchronization between the radar sensors is not matched, the two characteristics including an increase in the noise level and generation of the ghost target is repeatedly exhibited as shown in FIGS. 9 and 10.